Nonlinear grassland responses to past and future atmospheric CO2
Authors: Gill, RA, HW Polley, HB Johnson, LJ Anderson, H Maherali, RB Jackson
Carbon sequestration in soil organic matter (SOM) may moderate increases in atmospheric CO2 concentrations (Ca) as Ca increases to >500 Âµmol mol-1 this century from interglacial levels of <200 Âµmol mol-1. However, such storage depends on feedbacks between plant responses to Ca and nutrient availability. Here we present evidence that soil carbon storage and nitrogen cycling in a grassland ecosystem are much more responsive to increases in past Ca than to those forecast for the coming century. Along a continuous gradient of 200 to 550 Âµmol mol-1 Ca, increased Ca promoted higher photosynthetic rates and altered plant tissue chemistry. Soil C was lost at subambient Ca, but was unchanged at elevated Ca where losses of old soil C offset increases in new C. Along the experimental gradient in Ca there was a non-linear, three-fold decrease in N availability. The differences in sensitivity of C storage to historical and future Ca and increased nutrient limitation suggest that the passive sequestration of C in soils may have been important historically, but the ability of soils to continue as sinks is limited.